The present invention relates to a press device for a paper machine, or the like, having a shoe press roll. A shoe press "roll" comprises a stationary support member on which a press shoe is supported for radial displacement toward and away from, and which can be pressed hydraulically against a backing roll to define a web dewatering press nip between the rolls. More particularly, the invention concerns return means for urging the press shoe back from the backing roll.
Such a press device is known from Federal Republic of Germany Patent Publication 44 02 595 A1. The known press device is a so called long nip press device having a shoe press roll which can be pressed hydraulically against a backing roll. The shoe press roll includes a press shoe which is pressed by hydraulic means against the backing roll via a pressure chamber which extends in the length direction of the press shoe, i.e., across the width of the press device. The pressure chamber is formed by the press shoe at the radial outside of the chamber and a shoe bed beneath the shoe which rests on a stationary support member of the shoe press roll. A flexible material press jacket or shell, which is hydrodynamically lubricated in order to minimize its friction on the press shoe, extends in known manner over the press shoe and passes through the press nip.
To withdraw the press shoe into its starting position upon the absence of pressure in the pressure space and also to prevent the press shoe from falling down when the shoe press roll is arranged on top of the backing element, there are return elements which are acted on by springs and which act on the press shoe via pulleys. These are arranged within the pressure chamber.
This provides the flattest possible construction so that the dimensions of the press shoe unit, and particularly of the shoe bed, in the radial direction of the roll, need not be excessively enlarged.
A similar press device is known from Federal Republic of Germany Publication 41 13 623 C1 (also U.S. Pat. No. 5,223,100). Spring elements which counteract the pressure prevailing in the pressure chamber are also arranged within the pressure chamber. When the pressure chamber is almost without pressure, the spring elements return the press shoe in the direction toward the shoe bed.
One disadvantage of the above noted structural units is that corresponding lead throughs are necessary in the shoe bed. These require additional seals. Furthermore, holes are necessary in the support member, which correspondingly weakens it.
Both known press devices have the disadvantage that mounting of the press shoe together with the return devices is relatively expensive and the press shoe can be replaced only at great expense. Furthermore, even when the return elements are developed as spring elements which cooperate with cables conducted over pulleys, the pressure chamber is noticeably enlarged which correspondingly weakens the cross section of the support member. This promotes greater sagging of the shoe press roll, which may require compensation by additional measures, such as strengthening the support member.
Another disadvantage of the known press devices results from the press shoe being hydraulically acted upon only via a single pressure chamber which extends in the lengthwise direction of the press shoe across the width of the press device. Thus, the application force is the same over the entire width of the roll. However, in such press devices, the two edges of the paper web which travels through the press device may be more strongly dewatered than other regions across the paper web, particularly when a paper web of somewhat smaller than normal width is produced. In this case, the pressing force in the region of the edges of the web must be reduced.
Federal Republic of Germany Application 43 19 323 A1, discloses applying the press shoe to the press jacket, not using a single lengthwise pressure chamber across the width of the press roll but, instead, using a plurality of hydraulic cylinder/piston units arranged in a row in the lengthwise direction of the press shoe. This enables the application pressure of the press shoe in its lengthwise direction to be adapted to required circumstances.
Spring elements are arranged within the cylinder/piston units in order to assure pressing of the corresponding cylinder/piston unit against the press shoe even when the pressure chamber is without pressure. Even when there is not yet sufficient pressure in the pressure chamber, clean application of the sealing surfaces of the cylinder/piston unit against the press shoe should be assured, so that lateral emergence of hydraulic oil is avoided. The cylinder/piston units are tiltable by a certain amount to make their application against the press shoe possible.
Although the known press device permits a finer adjustment of the pressing pressure along the length of the press shoe across the width of the web, no measures are taken to assure the return of the press shoe in the direction toward the support member in opposition to the action of the spring force. Only limiting elements limit the maximum stroke of the cylinder/piston units. However, this is frequently insufficient, particularly if the shoe press roll is arranged on the top or above the backing roll, particularly upon the starting up of the paper machine and upon maintenance and adjustment work.
A press device of similar type is known from the publication EP 0 345 501 B1. However, it has the same disadvantages as the above press device.